python change for_loop in list comprehesion

Question

I've got the code as follows:

ix=0
list_test=[]
for el in parse_data_2['var_nm_final']:    
    if el=='URI':
        ix+=1
        list_test.append(ix)
    else:
        list_test.append(None)

parse_data_2 is my DF. As an output I would like to receive list with incremented value of ix or None depending on my condition. Meaning something like this

1
None
None
None
None
2
None
3
None
None
None
None
4

... etc.

I've tried to convert this loop to list comprehesion like this:

[ix+=1 if el=='URI'else None for el in parse_data_2['var_nm_final']]

but got error

[ix+=1 if el=='URI'else None for el in parse_data_2['var_nm_final']]
       ^
SyntaxError: invalid syntax

Could you explain me the issue with my code?

Personally I would use the explicit loop in this case. It's clear what it does, it's extremely easy to read, and it does what you want. — 2e0byo
– 2e0byo, Commented Nov 9, 2021 at 15:59

tobias_k · Accepted Answer · 2021-11-09 15:59:10Z

4

Use next on an itertools.count (or just range with generous upper bound):

>>> parse_data_2 = {'var_nm_final': ["URI", "foo", "bar", "URI", "URI", "blub", "URI"]}
>>> import itertools
>>> cnt = itertools.count(1)
>>> [next(cnt) if el == "URI" else None for el in parse_data_2["var_nm_final"]]
[1, None, None, 2, 3, None, 4]

answered Nov 9, 2021 at 15:59

tobias_k

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1 Comment

data_b77 Over a year ago

Awsome, didn't know it. thx

Joshua Voskamp · Accepted Answer · 2021-11-09 15:59:13Z

1

This is a great use-case for the walrus operator! But please, don't do this. Whatever you're trying to accomplish, this can almost certainly be done in a pythonic and performant manner.

[(ix:=ix+1) if el=='URI'else None for el in parse_data_2['var_nm_final']]

edited Nov 9, 2021 at 15:59

answered Nov 9, 2021 at 15:54

Joshua Voskamp

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5 Comments

2e0byo Over a year ago

well.. it's a usecase, I grant that ;)

Marco Bonelli Over a year ago

Please never write code like this...

Joshua Voskamp Over a year ago

@MarcoBonelli I hear you, this is terrible code. That said, it does what OP asked for...

data_b77 Over a year ago

What are disadvantages of this code?

Joshua Voskamp Over a year ago

@data_b77 whatever you're trying to do with your el variable can likely be accomplished by something like parse_data_2['var_nm_final'].eq('URI').sum() or similar -- generating a list comprehension with side-effects on your element generator is poor form, and likely not the most performant solution either.

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